The invention resides in a gas inlet for producing a directional and cooled gas jet in an ion source or a UV/fluorescence measuring cell.
It has been common practice to introduce a gas to be analyzed into the ion source of a mass spectrometer in an effusive manner. For this purpose, an admission duct (for example, the end of a gas-chromatographic capillary) extends into the ion source. The ion source may be of closed design (for example, many CI- or EI-ion sources for quadra-pole- or sector field mass spectrometers) or of an open design (for example, many ion sources for travel time mass spectrometers). In ion sources of closed design an area of the ion source is xe2x80x9cfloodedxe2x80x9d with inlet gas so that the atoms or molecules introduced partially impinges on the walls of the ion source before they are isolated and detected in the mass spectrometer. Ion sources of open design for IOF mass spectrometers are more suitable for the employment in connection with atom- or molecular beam techniques. In that case, a relatively directed gas jet is conducted through the ion source so that, in an ideal manner, it has little interaction with the structural components of the ion source.
In the travel time mass spectroscopy, effusive molecular beams [2], as well as skimmed [1] and non-skimmed [3, 4] supersonic molecular beams are used for that purpose (in each case pulsed or continuous (cw)). Supersonic molecular beams inlet systems provide for a cooling of the analysis gas in a vacuum by adiabatic expansion. It is however disadvantageous in present systems that the expansion needs to occur relatively remote from the location of the ionization. Since the density of the expansion gas jet (and, as a result, the ion yield for a given ionization volume) decreases with the distance from the expansion nozzle in square, the achievable sensitivity is limited.
Effusive molecular beam inlet systems permit a cooling of the sample. However, gas inlet systems for effusive molecular beams cannot be so constructed that the gas discharge is directed directly to the ionization location by way of a metallic needle, which extends into the center of the ion source [2]. A certain electric potential is applied to the needle in order to avoid disturbance of the withdrawal fields in the ion source. The needle needs to be heated to relatively high temperatures in order to prevent condensation of the low-volatile analyte molecules in the needle. In this connection, it has to be taken into consideration that the coldest point should not be at the needle tip. The necessary heating of the needle is problematic since the needle must be electrically insulated with regard to all the other parts of the device (for example, by a transition piece of ceramic material). Electric insulators are generally also thermal insulators and provide for only a small heat flux of, for example, the heated duct to the needle. Heating of the needle by electric heating elements or infrared radiators is also difficult since the needle extends between the withdrawal plates of the ion source.
The selectivity of the resonance ionization by lasers (REMPI) depends on the inlet system used because of the different cooling properties of the various systems. Aside of the effusive molecular beam inlet system (EMB) which may be used, among others, for the detection of whole substance classes, it is possible to ionize highly selectively and partially even isomer-selectively by using a supersonic molecular beam inlet system (jet). With the common supersonic nozzles developed for spectroscopic experiments, the utilization of the sample amount (that is, the measuring sensitivity that can be achieved) is not a limiting factor. Furthermore, the existing systems are not designed to avoid memory effects. For the application of REMPI-TOFMS spectrometers for analytical applications, the development of an improved jet inlet technique would be advantageous. Care has to be taken that the valves consist of inert materials in order to avoid memory effects or chemical decompositions (catalysis) of the sample molecules. Furthermore, the inlet valves should not have any dead volumes. It is also necessary that the valves can be heated to temperatures of more than 200xc2x0 C. so that also compounds of low volatility with a mass range greater than 250 amu are accessible. In addition, as little sensitivity as possible should be lost by the jet arrangement as compared with the effusive inlet technique. This can be achieved mainly by a more effective utilization of the sample entered in comparison with the jet arrangements used so far.
This increase can be achieved for example in that each laser pulse reaches the largest possible part of the sample. Since the excitation volume is predetermined by the dimensions of the laser beam (a widening of the laser beam would reduce the REMPI effective cross-section which is scaled for example with a two photon ionization with the square of the laser intensity) the spatial overlap of the molecular beam (jet) and the laser beam must be optimized. This can be realized, for example, by a pulsed inlet. Boesl and Zimmerman et al., disclose for example a heatable pulsed jet valve for analytical applications, for example for a gas chromatography jet REMPI coupling with minimized dead volume [5].
Pepich et al. discloses a GC supersonic molecular jet coupling for the laser-induced fluorescence spectroscopy (LIF), wherein the duty cycle is increased over the effusive inlet by the pulsed admission and by sample compression [6].
It is the object of the present invention to provide a gas inlet of the type referred to initially which facilitates an effective cooling of a continuous gas jet with a relatively low inlet flow volume employing simple design means.
In an arrangement for producing a directional and cooled gas jet in an ion source with a gas inlet or a UV/fluorescence detection cell including a gas inlet, wherein a capillary extends with one end into the interior of the ion source which is evacuated, the one end is provided with a nozzle for discharging a gas sample into the ion source while being subjected to adiabatic cooling and the width of the nozzle opening is at most 40% of the inner diameter of the capillary and the capillary is heatable for preventing condensation of gas sample components in the nozzle.
With respect to the state of the art, the device according to the invention has the following specific advantages:
The supersonic molecular beam expansion can be selected so as to occur directly in the ion source. In this way, in principle, the highest possible density, of the gas jet 4 is achieved at the ionization location. Special advantages of the gas admission reside in the fact that the sample is cooled adiabatically, the capillary can easily be heated up to its lower end, that is its tip, and a very simple design without movable parts is achieved. The device can be so designed that the sample molecules come in contact only with inert materials. By adjustment of the appropriate parameters (gas pressure) the cooling of the gases can be realized by an adiabatic expansion into the vacuum of the mass spectrometer, (supersonic molecular jet 4), wherein generally the continuous gas flow into the ionization chamber corresponds about to that of a continuous effusive inlet (see [7]). The flow rates of effusive inlet systems are typically in the range of 0.1-100 ml/min (1 bar). In comparison with an effusive capillary inlet, in the gas inlet according to the invention the stronger orientation of the supersonic molecular jet 4 is advantageous since a better overlapping of the laser beam and the gas jet can be achieved (higher sensitivity). Particularly with the gas inlet of the type referred to earlier, a continuous, cooled gas jet can be generated also at low gas flows ( less than 10 ml/min). As shown in FIG. 3, this can be achieved very well with the embodiment shown in FIG. 1B. A cooling of the inlet gas is advantageous for many mass spectrometric tasks. The small internal energy of cooled molecules often results in a reduced fragmentation degree in the mass spectrum. The cooling is particularly advantageous for the application of the resonance ionization with lasers (REMPI). With the use of a so-called supersonic molecular jet inlet system (Jet) for the cooling of the gas jet, it is possible with REMPI to ionize in a highly selective manner (partially even in an isomer-selective manner [1, 9]. Since the cooling occurs by the expansion, the sample gas admission conduit, the capillary 1 and the expansion nozzle 2 can be heated without detrimentally affecting the cooling properties. This is important for analytical applications. Without sufficient heating, sample components can condense in the supply conduit or in the gas inlet. An important application for the invention is the transfer of a chromatographic element of a continuous sample gas flow from an on-line sampling device into a cooled supersonic molecular jet 4. The inlet system described herein makes it possible that the expansion step occurs in the ion source of the mass spectrometer. In this way, the ions can be generated close to, or closely below, the expansion nozzle 2, which is very advantageous for the achievable detection sensitivity.